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What follows is an update on this issue.
The Problem:
Symantec Endpoint Protection 11.0.4 (Network Threat Protection)
periodically corrupts outbound UDP packets that are subject to
fragmentation. In addition, any version of SEP 11.0.x including
11.0.5 results in performance degradation when UDP packets are
fragmented.
Why does data corruption not affect OpenAFS/Win 1.5.33 and earlier?
All versions of OpenAFS for Windows 1.3.50 to 1.5.33 included
by default a cap on the MTU size used for sending and receiving
packets within the Rx library. This cap was configured using the
TransarcAFSDaemon\Parameters "RxMaxMTU" value. A value of 1260
was used to permit OpenAFS communications to work in conjunction
with Cisco IPSec VPN 4.x clients.
Why does data corruption affect OpenAFS/Win 1.5.34 through 1.5.68?
Cisco VPN Client 5.0 fixed the problems with OpenAFS. By the time
1.5.34 was released, 5.0 had been out for a significant period of
time and was widely distributed within the OpenAFS community.
It was determined that the >10% performance hit caused by not
sending full UDP packets was no longer required by the default
installation.
If the Windows network interface MTU is set to 1300, why does
OpenAFS continue to use 1444?
OpenAFS for Windows uses the IP Helper Library GetAdapterAddresses()
function to obtain the configuration data for all of the
installed network interfaces. This API fails to report the
static MTU limit restriction if one is configured in the registry.
As a result, OpenAFS for Windows is unaware that the network
interface will fragment UDP packets larger than the assigned value.
How is this problem being addressed in 1.5.69?
Asanka Herath (Secure Endpoints) contributed a patch to search
for a registry assigned MTU value for each network interface which
permits OpenAFS for Windows to report the correct MTU values to
the Rx library and in response to the "cmdebug <host> -addrs"
query. http://gerrit.openafs.org/#change,1105
In addition, a patch to the Rx library ensures that the interface
MTU values are used to limit packet size on sends as well as
on receives. http://gerrit.openafs.org/#change,1107
When these two patches are applied, packet fragmentation will
not occur. As a result the performance degradation and data
corruption caused by Symantec Endpoint Protection will be avoided.
Acknowledgments
Thanks to Richard Brittain and his management at Dartmouth for
the significant quantity of his time that was dedicated over the last
two months to help narrow down the root cause of this problem. Without
their efforts, the gatekeepers could not have resolved this issue on
behalf of the community.
Jeffrey Altman
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